U.S. Pat. No. 4,419,190 discloses a method according to which the internal resistance is measured with the aid of an alternating current passed through the probe. The alternating voltage component is separated from the direct voltage component of the probe and the internal resistance is determined from alternating current and alternating voltage. The frequency used is preferably about 5,000 Hz. From the difference between the measured internal resistance and a desired value of the internal resistance, a control deviation is formed with the aid of which heating of the probe is controlled. The disadvantage of this method for measuring the internal resistance is that a separate alternating current source and a separate alternating voltage evaluating circuit are required. An advantage is that no burden is placed on the probe voltage by the determination of the internal resistance, with the result that the internal resistance can be determined at very short time intervals.
U.S. Pat. No. 4,742,808 discloses a method for determining the internal resistance of a lambda probe, according to which the probe is periodically loaded by electrically connecting a load resistor. The internal resistance is calculated with the aid of the load resistor from the voltages measured in the loaded condition and in the unloaded condition. The value of the load resistor is compared to a threshold value to determine whether the probe is ready for operation. This method requires minimal circuit complexity since only the load resistor and a switch are required. A disadvantage is that the probe voltage recovers only relatively slowly after loading, with the result that the internal resistance cannot be measured by this method when the lambda probe is involved in control operations. In that case, the probe voltage must be measured approximately every 10 to 15 ms. However, after loading, the probe voltage only recovers in a time span of about 100 ms.